Devices with cannula and electrode lead for brain stimulation and methods of use and manufacture

ABSTRACT

A device for brain stimulation includes a cannula configured and arranged for insertion into a brain of a patient; at least one cannula electrode disposed on the cannula; and an electrode lead for insertion into the cannula, the electrode lead comprising at least one stimulating electrode.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/241,156, filed Sep. 30, 2005, which is incorporated herein byreference.

FIELD

The invention is directed to devices and methods for brain stimulationincluding deep brain stimulation. In addition, the invention is directedto devices and methods for brain stimulation using a cannula with atleast one cannula electrode and an electrode lead with at least onestimulating electrode.

BACKGROUND

Deep brain stimulation can be useful for treating a variety ofconditions including, for example, Parkinson's disease, dystonia,essential tremor, chronic pain, Huntington's Disease, levodopa-induceddyskinesias and rigidity, bradykinesia, epilepsy and seizures, eatingdisorders, and mood disorders. Typically, a lead with a stimulatingelectrode at or near a tip of the lead provides the stimulation totarget neurons in the brain. Magnetic resonance imaging (MRI) orcomputerized tomography (CT) scans can provide a starting point fordetermining where the stimulating electrode should be positioned toprovide the desired stimulus to the target neurons. To more preciselydetermine the target location, a recording lead with a recordingelectrode at or near the tip of the recording lead can be inserted intothe brain of the patient, and physiological maps can be generated.Typically, the recording lead is guided to the target location withinthe brain using a stereotactic frame and microdrive motor system.

As the recording lead is moved through the brain, field voltages andsingle unit voltages are observed with the recording electrode.Observation with the electrode (i.e., physiological mapping) may includeactivating the target neurons to generate electrical signals that can bereceived by the recording electrode. The mapping approach may alsoinclude electrical stimulation via the electrode that is also used forrecording. Once the position of the target neurons is determined, therecording lead can be removed and the stimulating lead inserted. Theobject of using the recording lead followed by insertion of thestimulating lead is to position the stimulating lead as near as possibleto the target neurons. The precise insertion of the stimulating lead andpositioning of the stimulating lead in the precise location indicated bythe recording lead can be particularly difficult. In some instances,multiple insertions of the recording lead are used for mapping, andmultiple insertions of a stimulating lead may need to occur to properlyposition the stimulating electrode.

BRIEF SUMMARY

One embodiment is a device for brain stimulation that includes a cannulaconfigured and arranged for insertion into a brain of a patient; atleast one cannula electrode disposed on the cannula; and an electrodelead for insertion into the cannula. The electrode lead includes atleast one stimulating electrode.

Another embodiment is a method of stimulating a portion of a brain of apatient. A cannula is inserted into the brain of the patient. Thecannula includes at least one cannula electrode. Tissue to be stimulatedis identified using the cannula electrode(s). An electrode lead isinserted into the cannula prior to, during, or after insertion of thecannula into the brain of the patient. The electrode lead includes atleast one stimulating electrode. The cannula is removed from the brainleaving the electrode lead in place. The tissue is stimulated using thestimulating electrode(s).

Yet another embodiment is a cannula for insertion of an electrode leadto stimulate brain tissue. The cannula includes a cannula bodyconfigured and arranged to receive the electrode lead and a plurality ofcannula electrodes disposed on the cannula body.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following drawings. In the drawings,like reference numerals refer to like parts throughout the variousfigures unless otherwise specified.

For a better understanding of the present invention, reference will bemade to the following Detailed Description, which is to be read inassociation with the accompanying drawings, wherein:

FIG. 1 is a schematic perspective side view of one embodiment of acannula, according to the invention;

FIG. 2 is a schematic side view of one embodiment of an electrode lead,according to the invention;

FIG. 3 is a schematic side view of the electrode lead of FIG. 2 insertedin the cannula of FIG. 1;

FIG. 4 is a schematic side view of one embodiment of a recordingelectrode arrangement, according to the invention;

FIG. 5 is a schematic side view of one embodiment of a stimulatingelectrode arrangement, according to the invention;

FIG. 6 is a schematic side view of one embodiment of a recordingelectrode and stimulating electrode arrangement, according to theinvention;

FIG. 7 is a schematic side view of a lead and associated hardware forinsertion into a cranium, according to the invention;

FIG. 8 is a schematic side view of one embodiment of a lead with animplantable pulse generator unit, according to the invention; and

FIG. 9 is a schematic block diagram of one embodiment of an implantablepulse generator unit, according to the invention.

DETAILED DESCRIPTION

The present invention is directed to the area of devices and methods forbrain stimulation including deep brain stimulation. In addition, theinvention is directed to devices and method for brain stimulation usinga cannula with at least one cannula electrode and an electrode lead withat least one stimulating electrode.

Existing techniques of brain stimulation often use multiple penetrationsinto the brain to locate the tissue to be stimulated and to position thestimulating electrode. Each penetration is associated with a possibilityof hemorrhage. In addition, using one lead for identifying the desiredtissue for stimulation and a second lead for stimulating the tissue canresult in errors associated with the placement of the second lead.Sources of error can include, for example, errors associated with thestereotactic apparatus; slipping into a previous penetration tract whenseveral tracts have been made in the effort to find the tissue to bestimulated; and displacement of tissue with each penetration. Because ofthese concerns, some clinical centers do not use recording electrodes toidentify the tissue to be stimulated and, therefore, have a lowerprobability of accurately placing the electrode lead with thestimulating electrode. Further, each penetration with a microelectrodeunder current paradigms can increase the cost of surgery by extendingthe operating room time and including additional time from a neurologistto aid in physiological mapping.

A device for deep brain stimulation can include a cannula with one ormore cannula electrodes and an electrode lead, which is inserted intothe cannula, that contains one or more stimulating electrodes. Thecannula electrodes in the cannula can be used to identify the tissue tobe stimulated and then the cannula can be removed leaving the electrodelead with the stimulating electrode(s) in place. This allows apractitioner to determine the position of the target neurons using thecannula electrode(s) and position the stimulating electrode(s)accordingly without removal of a recording lead and insertion of aseparate stimulation lead. The cannula can include cannula electrodesspaced around the circumference of the lead to more precisely determinethe position of the target neurons. The cannula electrodes can be usedas recording electrodes, stimulating electrodes, or both, if desired. Inaddition, it will be recognized that stimulating electrodes may be usedas recording electrodes and recording electrodes may be used asstimulating electrodes.

FIG. 1 illustrates one embodiment of a cannula 110, FIG. 2 illustratesone embodiment of an electrode lead 120, and FIG. 3 illustrates thecannula and electrode lead together. The cannula 110 includes one ormore cannula electrodes 112 and defines a lumen 114 through which theelectrode lead 120 can be inserted. As illustrated in FIG. 7, thecannula 110 also includes conductors 116 disposed within the cannula orin the lumen that extend from each of the cannula electrodes 112 out ofthe cannula to a control unit 118.

The cannula 110 can be formed of any non-conducting material such as,for example, a plastic material. Preferably, the cannula 110 is formedof a substantially rigid material (e.g., sufficiently rigid forinsertion in the brain without buckling) which facilitates insertion ofthe cannula into the brain of the patient and positioning of the cannulaand electrode lead near the tissue to be stimulated. In someembodiments, the cannula 110 is a disposable unit so that it isdiscarded after use and does not need to be subsequently sterilized forreuse.

The cannula electrode(s) 112 can be made using a metal, alloy,conductive oxide, or other conductive material. Examples of suitablematerials include platinum, iridium, platinum iridium alloy, stainlesssteel, titanium, and tungsten.

Any type of electrode can be used for the cannula electrodes includingmonopolar recording electrodes, bipolar recording electrodes (see FIG.4), other multipolar recording electrodes, and any type of stimulatingelectrode arrangement. In at least some embodiments, bipolar or othermultipolar recording electrodes are preferred because they can assist infinding nearby electrical signals, while disregarding or reducingdistant electrical signals by observation of the differential betweenthe signals from the two or more, closely-spaced electrodes. It will berecognized that the cannula electrodes can all be the same or thatdifferent types of cannula electrodes can be used on a single cannula.

Any cannula electrode configuration can be used including electrode padsor plates. A preferred cannula electrode for at least some embodimentsis a tip of a wire. This type of electrode can assist in more preciselocation of the target neurons because of the small surface area fordetection of electrical signals. Such cannula electrodes may have adiameter of no more than 200 μm or no less than 1 □m. The diameter maybe in the range from, for example, 25 μm to 100 μm. In one embodiment,the cannula electrodes 112 correspond to wire conductors that extend outof the cannula 110 and are then trimmed or ground down flush with thecannula surface.

In at least some embodiments, cannula electrodes 112 are arranged atvarious positions around the lateral circumference of the cannula 110.In these arrangements, the cannula electrodes are positioned inirregular or, preferably, regular intervals around the cannula. Forexample, in FIG. 1, the cannula electrodes 112 are positioned in ringsaround the cannula with about 90° separation between neighboring cannulaelectrodes of a particular ring. In other embodiments, the cannulaelectrodes 106 are positioned around the lead with about 60°, 72°, 120°,180°, or any other angular separation between neighboring cannulaelectrodes. In addition, cannula electrodes can be formed in one or morerings around the circumference of the cannula 110. FIG. 1 illustratesone embodiment with two rings. FIG. 4 illustrates an embodiment withpairs of recording electrodes 112 (e.g., bipolar recording electrodes)forming a single ring around the circumference of the cannula.Positioning the cannula electrodes 112 around the cannula 110 in thismanner can assist in determining the position of the target neuronsbecause the cannula electrodes, individually or in pairs (or any othergrouping), can sample the brain tissue around the cannula withoutrotating the cannula. In another embodiment, electrodes can be disposedin a helical arrangement around the cannula. The cannula can then berotated to sample the tissue using the electrodes.

The cannula is hollow with a central region 114 that receives theelectrode lead 120. Preferably, the electrode lead 120 slides into thecannula relatively easily and the cannula can also be slid off theelectrode lead once the correct tissue has been located. Generally, whenthe electrode lead is inserted in the cannula, the stimulatingelectrodes of the electrode lead are aligned with the cannula electrodesof the cannula, as illustrated in FIG. 3. Angular alignment ofstimulating electrodes with cannula electrodes may also be practiced ifthe stimulating electrodes are not ring-shaped. Preferably, the relativealignments of the contacts on the cannula and on the electrode lead arealways known, for example, by holding the electrode lead firmly so thatthe relative alignments are not accidentally altered. In someembodiments, markings or other indicia are provided on the electrodelead and/or cannula to facilitate alignment of the electrode lead withthe cannula. The electrode lead 120 typically includes one or morestimulating electrodes 122 disposed on the lead for stimulating thetarget tissue. In at least some embodiments, the electrode lead 120 issubstantially less rigid than the cannula 110 to reduce or avoid damageto the brain tissue with extended implantation and use of the lead. Theelectrode lead 120 can be formed of a non-conducting material such as,for example, a polymeric material. Suitable polymeric materials include,for example, silicone rubber and polyethylene. Preferably, the lead ismade using a biocompatible material. In at least some instances, thelead may be in contact with body tissue for extended periods of time.

In at least some embodiments, the electrode lead 120 has across-sectional diameter of, for example, no more than 1.5 mm and thediameter may be in the range of 1 to 1.2 mm. The lead may have a lengthof, for example, at least 10 mm and the length of the lead may be in therange of 10 to 120 mm.

The electrode lead 120 includes one or more stimulating electrodes 122arranged along the longitudinal axis of the lead, preferably, near adistal end of the lead. In at least some embodiments, the lead includesa plurality of stimulating electrodes. A conductor is attached to eachstimulating electrode 122 and exits the electrode lead for connection toa control unit 128. In at least some embodiments, the stimulatingelectrodes have a surface area of at least 0.1 mm² or at least 5 mm².The surface area may be in the range from, for example, 0.1 mm² to 12mm².

In some embodiments, a stimulating electrode 122 forms a ring that fullyor substantially encircles the lead 120, as illustrated in FIGS. 2 and3. In other embodiments, the stimulating electrodes are not rings, butare instead discrete shapes disposed on one side (or multiple sides) ofthe lead, as illustrated, for example, in FIGS. 5 and 6. A variety ofshapes can be used for the stimulating electrodes including, forexample, rings, circles, ovals, squares, rectangles, triangles, etc.Stimulating electrodes 112 can be positioned around the circumference ofthe lead 120 in a similar manner to that described for the cannulaelectrodes.

The electrode lead 120 may also include one or more recording electrodesdisposed on the lead. Optionally, one or more of the recordingelectrodes can be positioned within one or more of the stimulatingelectrode using an arrangement such as that illustrated in FIG. 6. Inthis arrangement, there is a nonconducting region 124 separating thestimulating electrode 122 and the recording electrode 112. Othersuitable electrode arrangements are described in U.S. patent applicationSer. No. 11/030,546, filed Jan. 5, 2005, incorporated herein byreference.

The stimulating electrodes can be made using a metal, alloy, conductiveoxide or other conductive material. Examples of suitable materialsinclude platinum, iridium, platinum iridium alloy, stainless steel,titanium, or tungsten. Preferably, the stimulating electrodes are madeof a material that is biocompatible and does not substantially corrodeunder expected operating conditions in the operating environment for theexpected duration of use.

Conductors that attach to the stimulating electrode(s) 122 also passthrough the electrode lead 120. The conductors 126 continue to connectto a control unit 128 (see FIG. 7). The control unit 128 providesstimulation signals, often in the form of pulses, to the stimulatingelectrodes 122. This control unit 128 can be the same as the controlunit 118 coupled to the cannula electrodes.

In one example of implantation of the electrode lead illustrated in FIG.7, access to the desired position in the brain can be accomplished bydrilling a hole in the patient's skull or cranium 206 with a cranialdrill (commonly referred to as a burr), and coagulating and incising thedura mater, or brain covering. The electrode lead 120 can be insertedinto the cranium and brain tissue with the assistance of the cannula110. The electrode lead can be placed in the cannula prior to,simultaneously with, or after insertion of the cannula into the brain ofthe patient. The cannula and electrode lead can be guided to the targetlocation within the brain using, for example, a stereotactic frame 204and a microdrive motor system 202, or a frameless microdrive system.

The cannula electrode(s) 112 on the cannula 110 can be observed using acontrol unit 118 attached to the conductors 116 extending from thecannula 110 to identify the target neurons. Once the target tissue isidentified, the cannula can be removed leaving the electrode lead andassociated stimulating electrode(s). The stimulating electrodes can thenbe activated to provide the desired stimulation to the target neurons bycoupling the stimulating electrode 122 to a control unit 128 viaconductors 126. The cannula electrodes can also be used to providestimulation. In some instances, the stimulation provided by the cannulaelectrodes is temporary and is unavailable when the cannula is removed.The control unit 128 can be the same or different from the control unit118 used with the cannula electrodes 112. The control unit 128 can alsobe used to operate any recording electrodes on the lead.

In some embodiments, measurement devices coupled to the muscles or othertissues stimulated by the target neurons or a unit responsive to thepatient or clinician can be coupled to the control unit or microdrivemotor system. The measurement device, user, or clinician can indicate aresponse by the target muscles or other tissues to the stimulation orrecording electrode(s) to further identify the target neurons andfacilitate positioning of the stimulating electrode(s). For example, ifthe target neurons are directed to a muscle experiencing tremors, ameasurement device can be used to observe the muscle and indicatechanges in tremor frequency or amplitude in response to stimulation ofneurons. Alternatively, the patient or clinician may observe the muscleand provide feedback.

FIG. 8 illustrates another embodiment that includes a lead 120 and animplantable pulse generator unit 220 coupled to the lead. The lead 120includes one or more stimulating electrodes 122 and, optionally, one ormore recording electrodes. The arrangement of these electrodes can beselected as described above. The stimulating electrode(s) 122 and,optionally, any recording electrodes, are coupled to the implantablepulse generator unit 220 by conductors running through the lead 120.

The implantable pulse generator unit 220 can be permanently ordetachably coupled to the lead 120. In some embodiments, the lead 120has a connector (not shown) that can be coupled to the implantable pulsegenerator unit 220 before, during, or after implantation of the leadinto the brain tissue 216. In one embodiment, the lead 120 is implantedas illustrated and discussed relative to FIG. 7. The implantable pulsegenerator is then coupled to the lead after implantation. In anotherembodiment, the implantable pulse generator is coupled to the leadduring implantation and the implantable pulse generator provides signalsfrom the recording electrodes to an external control unit to determinethe tissue to be stimulated. Once the lead is implanted and positioned,the implantable pulse generator is implanted.

The implantable pulse generator can be implanted in any convenientportion of the body including in the neck or behind the ear. In oneembodiment, the implantable pulse generator is implanted in the burrhole in the patient's skull 206 formed for insertion of the lead 120.Preferably, the implantable pulse generator does not extendsubstantially outside the exterior of the skull. Preferably, theimplantable pulse generator is adhesively attached to the skull and/or aplate is positioned over the burr hole and attached to the skull orscalp to keep the implantable pulse generator in place. Preferably, theimplantable pulse generator does not extend too far into the cranialcavity so that contact with brain tissue is avoided.

The implantable pulse generator unit 220 provides pulses of electricalenergy to the stimulating electrode(s) 122 to stimulate the desiredbrain tissue. In some embodiments, the implantable pulse generator unitcan also perform one or more other functions such as, for example,receiving signals from the recording electrodes; evaluating signals fromthe recording electrodes; altering or adjusting stimulation pulseparameters such as, for example, pulse frequency, pulse duration, pulsewaveform, and pulse strength, as well as determining which electrodessink and source the current comprising the pulse; transmittinginformation to an external control unit 222; receiving signals, such ascontrol signals or information, from an external control unit 222. Theimplantable pulse generator can include a power source 226, asillustrated in FIG. 9. Any power source can be used including, forexample, a battery such as a primary battery or a rechargeable battery.Examples of other power sources include super capacitors, nuclear oratomic batteries, mechanical resonators, infrared collectors,thermally-powered energy sources, flexural powered energy sources,bioenergy power sources, fuel cells, bioelectric cells, osmotic pressurepumps, and the like including the power sources described in U.S. PatentApplication Publication No. 2004/0059392, incorporated herein byreference.

As another alternative, power can be supplied by an external powersource through inductive coupling via an optional antenna 228. Theexternal power source can be in a device that is mounted on the skin ofthe user or in a unit that is provided near the patient on a permanentor periodic basis.

If the power source 226 is a rechargeable battery, the battery may berecharged using the optional antenna 228, if desired. Power can beprovided to the battery 226 for recharging by inductively coupling thebattery through the antenna to a recharging unit 224 external to thepatient.

A processor 232 is typically provided in the implantable pulse generatorto control the timing and electrical characteristics of the pulses sentto the electrodes. For example, the processor can, if desired, controlone or more of the timing, periodicity, strength, duration, and waveformof the pulses. Any processor can be used and can be as simple as anelectronic device that produces pulses at a regular interval or theprocessor can be capable of receiving and interpreting instructions froman external control unit 222.

In one embodiment, the antenna 228 is capable of receiving signals(e.g., RF signals) from an external control unit 222. The externalcontrol unit 222 can be device that is worn on the skin of the user orcan be carried by the user and can have a form similar to a pager orcellular phone, if desired. As another alternative, the external controlunit 222 may not be worn or carried by the user but may only beavailable at, for example, a home station or at a clinician's office.

The signals sent to the processor 232 via the antenna 228 and receiver230 can be used to modify or otherwise direct the operation of theimplantable pulse generator. For example, the signals may be used tomodify the pulses of the implantable pulse generator such as modifyingone or more of pulse duration, pulse frequency, pulse waveform, andpulse strength. The signals may also direct the implantable pulsegenerator to cease operation or to start operation or to start chargingthe battery. Additionally or alternatively, the implantable pulsegenerator can include a port into which a lead to the external controlunit can be plugged so that information, control signals, or the likecan be transmitted or received through a wired connection.

Optionally, the implantable pulse generator may include a transmitter(not shown) coupled to the processor and antenna for transmittingsignals back to the external control unit 222 or another unit capable ofreceiving the signals. For example, the implantable pulse generator maytransmit signals indicating whether the implantable pulse generator isoperating properly or not or indicating when the battery needs to becharged. The processor may also be capable of transmitting informationabout the pulse characteristics so that a user or clinician candetermine or verify the characteristics. In some embodiments, theimplantable pulse generator can send back signals to the externalcontrol unit from any recording electrodes on the lead 120. Such signalsmay be used to monitor the stimulation treatment, to verify that thelead is still correctly positioned, or to assist in the implantationprocedure.

The implantable pulse generator may include information storagecapacity. This can be used to store pulse parameters and the like, aswell as information that can be later transmitted to the externalcontrol unit 222.

The above specification, examples and data provide a description of themanufacture and use of the composition of the invention. Since manyembodiments of the invention can be made without departing from thespirit and scope of the invention, the invention also resides in theclaims hereinafter appended.

1. A method of stimulating a portion of a brain of a patient, the methodcomprising: inserting a cannula into the brain of the patient, thecannula comprising at least one cannula electrode; using the at leastone cannula electrode to find tissue to be stimulated, repositioning thecannula as needed to find the tissue to be stimulated; inserting anelectrode lead into the cannula, the electrode lead comprising at leastone stimulating electrode; removing the cannula from the brain leavingthe electrode lead in place; and stimulating the tissue using the atleast one stimulating electrode.
 2. The method of claim 1, wherein theelectrode lead is inserted into the cannula prior to inserting thecannula into the brain of the patient.
 3. The method of claim 1, whereinthe tissue is stimulated prior to removing the cannula from the brain.4. The method of claim 1, wherein inserting the electrode lead into thecannula comprises aligning the at least one stimulating electrode withthe at least one cannula electrode of the cannula.
 5. The method ofclaim 1, wherein using the at least one cannula electrode to find tissueto be stimulated comprises positioning the cannula within the brain andobserving a signal at the cannula electrodes.
 6. The method of claim 5,wherein using the at least one cannula electrode to find tissue to bestimulated further comprises repositioning the cannula at a secondposition within the brain and observing a signal at the cannulaelectrodes.
 7. The method of claim 1, wherein inserting the cannulacomprises inserting the cannula into the brain of the patient, whereinthe at least one cannula electrode comprises a plurality of cannulaelectrodes.
 8. The method of claim 7, wherein inserting the cannulacomprises inserting the cannula into the brain of the patient, whereinthe plurality of cannula electrodes are disposed around thecircumference of the cannula.
 9. The method of claim 8, whereininserting the cannula comprises inserting the cannula into the brain ofthe patient, wherein at least a portion of the plurality of cannulaelectrodes are disposed in regular intervals around the circumference ofthe cannula.
 10. The method of claim 1, wherein inserting the cannulacomprises inserting the cannula into the brain of the patient, whereinat least a portion of the cannula electrodes are disposedcircumferentially around the cannula to form a plurality of rings ofelectrodes around the circumference of the cannula body, wherein eachring of electrodes comprises at least two of the cannula electrodes andwherein the rings of electrodes are spaced apart from each other atdifferent lateral distances from a tip of the cannula body.
 11. Themethod of claim 1, wherein inserting the cannula comprises inserting thecannula into the brain of the patient, wherein the cannula furthercomprises a wall and at least one conductor coupled to the at least onecannula electrodes, wherein the at least one conductor is disposed in,and passes through, material forming the wall of the cannula.
 12. Themethod of claim 1, wherein inserting the electrode lead comprisesinserting the electrode lead into the cannula, wherein at least one ofthe stimulating electrodes of the electrode lead is not ring-shaped. 13.The method of claim 1, wherein inserting the cannula comprises insertingthe cannula into the brain of the patient, wherein at least a portion ofthe cannula electrodes are disposed in a helical arrangement around thecannula body.
 14. The method of claim 1, wherein the electrode lead issubstantially less rigid than the cannula.
 15. A cannula for insertionof an electrode lead to stimulate brain tissue, the cannula comprising:a cannula body configured and arranged to receive the electrode lead;and a plurality of cannula electrodes disposed on the cannula body. 16.The cannula of claim 15, wherein at least a portion of the cannulaelectrodes are disposed in regular intervals around a circumference ofthe cannula body.
 17. The cannula of claim 15, wherein at least aportion of the cannula electrodes are disposed in a plurality of ringsaround the circumference of the cannula body.
 18. The cannula of claim15, further comprising conductors disposed in the cannula body andcoupled to the plurality of cannula electrodes and extending out of adistal end of the cannula for connection to a controller.
 19. Thecannula of claim 15, wherein at least a portion of the cannulaelectrodes are disposed circumferentially around the cannula to form aplurality of rings of electrodes around the circumference of the cannulabody, wherein each ring of electrodes comprises at least two of thecannula electrodes and wherein the rings of electrodes are spaced apartfrom each other at different lateral distances from a tip of the cannulabody.
 20. The cannula of claim 15, wherein the cannula further comprisesa wall and at least one conductor coupled to the at least one cannulaelectrodes, wherein the at least one conductor is disposed in, andpasses through, material forming the wall of the cannula.